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This is an interesting question, particularly because the definition of "better" is undefined. What does it mean to be better?

We can make all sorts of arguments about thermal efficiency, and related engineering concepts, but the interesting thing is this: The Otto cycle is more commonly used in modest size engines (single cylinders for lawn mowers, etc. up through automobiles) while the Diesel cycle is more popular with larger engines (trucks, stationary power, etc.) What does it mean to be better?

In actual practice, "better" has many definitions. It may mean most cost efficient, it may mean least polluting, it may mean most readily available, etc. The whole key to the answer is in the question, "What do you mean by better?"

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Diesels have a higher thermal efficiency because, unlike the Internal Combustion engine, they rely solely on the heat produced from the high compression ratios to produce ignition for the power stroke. As DrD remarked, diesels are generally used to propel larger machines and vehicles because, volume for volume, they produce more torque at less rpm than the IC engine. In addition their fuel consumption is less and they are inherently more robust and reliable. I am currently working on a 3 cylinder 2 stroke turbocharged diesel engine intended to propel a new generation of aeroplanes

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IC (Internal Combustion) engines refer to both diesel and gas, what differs gas are called spark ignition engines and diesels are called compression ignition engines, they are both called IC engines, and you are right that diesels have a higher thermally efficiency. External combustion engines are like steam engines and Stirling engines. Diesels can burn fatty liquids like soybean oil, peanut oil, etc. while gas engines can only burn vapor, like wood gas, natural gas, propane, gasoline vapor, ethanol vapor, etc. Why did you only call gas ic and not diesels?

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Hi Joshua,

I've just read your message and I couldn't agree with you more. The Diesel engine and the gas engine ARE both internal combustion engines. It's a very amateurish mistake that I've made and I apologise to anyone who may have been misled by this. I can't imagine what I was thinking of when I wrote it. Thanks for your correction. 

Roger

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Diesel Engines are high compression Engines ' at the time of combustion the push on the Piston due to a higher compression resulting in the subsequent explosion of diesel mist in a robust structure ensures that the total thermal energy converts to work & is not dissipated as heat - hence more thermal efficiency 

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The reason being for same maximum pressure in deisel engine heat is being added at constant pressure where as in otto cyle heat is added at constant volume due to which during heat addition process in otto cycle there is rise in pressure where as in deisel engine there is no rise in pressure and during heat addition process pressure is same as achieved in compression ratio so the area in P-V diagram is more in deisel engine as compare to petrol engine due to which work done is more for same amount of heat addition.

I mean to say  that in deisel cycle to acheive same maximum pressure it is being achieved by high compression ratio where as in petrol engine to achieve that compression ratio  it is by compression ratio and heat of combustion. So high compression ratios in deisel engine enable them to achieve high efficiency for same maximum pressure.

Edited by Shaleen Rawat

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For same heat input and constant maximum temperature ,the heat rejection in case of Otto cycle is more than heat rejection in case of diesel cycle.so, the diesel cycle is thermally efficient than Otto cycle in this case.

Sorry,in place of maximum temperature,maximum pressure should be there.

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